The goal of this research project is to determine the molecular basis of the efficacy of the class of drugs known as the actinomycins in the inhibition of RNA synthesis. The study will center on actinomycin D, an important drug in both clinical and biochemical studies. We plan to use deoxydinucleotides and deoxytetranucleotides as models for DNA in studying the actinomycin D - DNA complex. Previous visible spectroscopic and proton nuclear magnetic resonance results have indicated that the dinucleotides serve as excellent model systems. The data has demonstrated that actinomycin has a preference for the dGpdC sequence. The sequence specificity and the geometry of the complexes are the subject of our present research. The experiments include both optical and nuclear magnetic resonance experiments on the complexes of actinomycin D with mixtures of complementary deoxydinucleotides and deoxytetranucleotides.